Rare earth metal oxysulfide regenerative material and regenerator

ABSTRACT

A rare earth metal oxysulfide represented by a general formula R 2 O 2 S (R denotes one kind or two or more kinds of rare earth elements to be selected from a group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, as wells as Y) is formed into spherical granules. The mean particle size of the granules is 0.05–1 mm and their relative density is 96% or over. The granules are used as a regenerative material at the liquid helium temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the national phase of PCT Application No.PCT/JP02/05933, filed Jun. 13, 2002, now Publication No. WO 02/103259,published Dec. 27, 2002, which corresponds to Japanese Application No.2002-10196, filed Jan. 18, 2002 and Japanese Application No.2001-183895, filed Jun. 18, 2001.

FIELD OF THE INVENTION

The present invention relates to a rare earth metal oxysulfideregenerative material and a regenerator using the material. Inparticular, the invention relates to a regenerative material and aregenerator both having a large heat capacity at a temperature in theneighborhood of 4.2 K required in the liquefaction of He gas, andgenerating no abrasion dust during a refrigerator operation.

PRIOR ART

Liquid helium is needed to cool superconducting magnets, sensors, etc.,and an enormous amount of compression work is needed to liquefy He gas,thus a large-sized refrigerator is needed for this purpose. It, however,is difficult to use large-sized refrigerators for small-sized facilitiesusing superconductive phenomena such as linear motor cars or MRI(magnetic resonance induction analyzer). Hence a small-sizedhigh-performance refrigerator that can generate the liquid heliumtemperature (4.2 K) is indispensable.

The cooling efficiency, the lowest achieved temperature, and the like ofa small-sized refrigerator depend on regenerative materials, the fillersof the regenerator. Such regenerative materials are required to have asufficiently large heat capacity and a high heat exchange efficiency forhelium or refrigerant passed through the regenerator. The conventionalmetallic regenerative materials such as Pb show a sharp drop in the heatcapacity at 10 K and under. Accordingly, some regenerative materialscomprising rare earth intermetallic compounds such as HoCu₂ or ErNi havebeen developed (JP2609747, U.S. Pat. No. 5,449,416). They have largeheat capacities at 20 K–7 K as shown in FIG. 1, but their heatcapacities are small at temperatures under 7 K. Moreover, regenerativematerials are required to have a good durability against heat impactsand vibrations during a refrigerator operation.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a regenerativematerial having a large heat capacity in the neighborhood of the liquidhelium temperature and a high durability against heat impacts andvibrations, and to provide a regenerator using the regenerativematerial.

The secondary object of the invention is to provide a regenerativematerial and a regenerator suited to refrigerating down to temperaturesof 4 K to 7 K.

Another secondary object of the invention is to provide a regenerativematerial and a regenerator suited to refrigerating down to temperaturesof 2 K to 4 K.

Another secondary object of the invention is to improve the durabilityof the regenerative material for the operation of regenerators.

Regenerative Material

The regenerative material of the present invention comprises a rareearth metal oxysulfide represented by a general formula R₂O₂S® denoteone kind or two or more kinds of rare earth elements to be selected froma group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, aswells as Y). Preferably, the rare earth element is at least one memberof a group comprising Gd, Tb, Dy, Ho and Er, and more preferably, therare earth element is Gd or Tb.

For example, when the rare earth metal oxysulfide is Gd_(2−x)Tb_(x)O₂S(x=0.2–2), the temperature at which a peak of its specific heat ispresented can be varied in a range of the neighborhood of 6 K to theneighborhood of 4 K. Especially, when the value of x is 1.6–2,preferably, 1.8–2, and more preferably, 1.9–2, a temperature at which apeak of its specific heat is presented is a rather higher than atemperature at which a peak of its specific heat is presented on Gd₂O₂S.When this regenerative material is combined with a regenerative materialincluding Gd as rare earth elements as the main component, largespecific heats can be presented for a broad range of 4 K to 7 K, andthis combination of regenerative materials is particularly suited for acooling down to the neighborhood of the liquid helium temperature.

As oxysulfides of Ho or Dy have their specific heat peaks on the lowertemperature side of those of oxysulfides of Gd, the former areparticularly suited for a refrigerating down to 2 K–4 K. And, forexample, when a regenerative material including Gd as the main componentof its rare earth elements is arranged on the higher temperature side,and a regenerative material comprising an oxysulfide of Ho or Dy isarranged on the lower temperature side thereof, refrigerating down to 4K can be done by the regenerative material including Gd as the maincomponent of the rare earth elements, and refrigerating at 4 K and undercan be done by the regenerative material including Ho or Dy as the maincomponent of the rare earth elements; thus the refrigerating down to 4 Kand under can be done efficiently.

Preferably, the rare earth metal oxysulfides are used in the form ofgranules, and more preferably, the mean particle size of the granules is0.05 mm–1 mm. The mean aspect ratio of the granules is preferably 3 orunder, and the relative density of the granules is preferably 96% orover. The rare earth metal oxysulfide in the granules has preferably amean grain size of 100 μm or under, the surface roughness based on themaximum height Rmax of the granules is preferably 10 m or under, and anexcess sulfur content in the granules is preferably 10000 wtppm orunder. Preferably, a volume specific heat of the granules has themaximum at 2 K–7 K.

Addition of Toughenings

Preferably, an additive comprising an oxide, a carbide or a nitride ofat least one member of a group comprising alkaline-earth metals,transition metals, and elements of 3b and 4b groups of the periodic lawincluding B and excluding C is added to the rare earth metal oxysulfideby 0.05–30 wt %. The amount of addition is expressed in such a mannerthat one part by weight of the additive in 100 parts by weight of theregenerative material is 1 wt %.

Preferably, the additive is at least one member of a group comprisingAl₂O3, ZrO₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiC and TiC, and morepreferably, the additive is at least one member of a group comprisingZrO₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiC, and TiC. Mullite is acompound with xAl₂O3.ySiO₂ composition (x:y=3:2–2:1), and Sialon is anon-stoichiometric compound of Si, Al, O, and N.

Preferably, the additive is an oxide of at least one alkaline-earthmetal element of a group comprising Mg, Ca, Sr, and Ba.

Preferably, the additive is an oxide of at least one transition metalelement of a group comprising elements of which atomic numbers are 22(Ti)–31 (Ga) and 72 (Hf).

Preferably, R₂O₂S phase as the main phase and a second phase containingthe additive and differing from the main phase are formed in theceramics microstructures of the rare earth metal oxysulfide regenerativematerial.

Regenerator

A rare earth metal oxysulfide regenerative material is packed in anappropriate cylinder or the like to make a regenerator. The rare earthmetal oxysulfides exhibit good specific heats at 7 K or under, and whena regenerative material including HoCu2 as a main component is arrangedon the higher temperature side of a rare earth metal oxysulfideregenerative material, refrigerating down to 7 K can be done by HoCu₂and refrigerating at 7 K and under can be done by the rare earth metaloxysulfide regenerative material; thus refrigerating down to 7 K andunder can be done efficiently.

Oxysulfide regenerative materials including Gd as the main component ofrare earth elements thereof have their specific heat peaks at 5 K–4 K,and their specific heats at 7 K–5 K are not sufficient. Hence,preferably, an oxysulfide regenerative material including Tb as the maincomponent of rare earth elements thereof is arranged on the highertemperature side of an oxysulfide regenerative material including Gd asthe main component. As for refrigerating down to 4 K and under, anoxysulfide regenerative material including Gd as the main component ofrare earth elements thereof is arranged, and an oxysulfide regenerativematerial including Ho or Dy as the main component of rare earth elementsis arranged on the lower tempera side of the former. Refrigerating downto 2 K–4 K is effective, for example, for enhancement of the sensitivityof an X-ray detector with cooling on the analyze of semiconductors withtransmission X-rays or the like, and for the preliminary stage coolingof an adiabatic demagnetization refrigerator.

Now, the representation of rare earth elements in rare earth metaloxysulfides will be explained. An oxysulfide regenerative material of Gdor one with Gd being its main component means that 50 atom % or over ofits metallic components is Gd. For example, as shown in Table 1,Gd₁Tb₁O₂S has a specific heat peak on the lower temperature side of thatof Gd₂O₂S and is similar to Gd₂O₂S rather than to Tb₂O₂S. As for Tb, Dy,Ho, etc., an oxysulfide of any of these elements or an oxysulfide withany of these elements as the main component indicates that 80 atom % orover of its metallic components is one of these elements. For example,even if the metallic components of an oxysulfide is substituted with adifferent element by 10 atom %, the specific heat characteristics of theresulting oxysulfide will not differ much from those of the originalone.

Refrigeration Characteristics

Rare earth metal oxysulfide regenerative materials undergo magneticphase transition in the neighborhood of 7 K–2K, and they have heatcapacities as large as 2 to 5 times those of the conventionalregenerative materials such as HoCu₂ or ErNi. Accordingly, rare earthmetal oxysulfide regenerative materials show high refrigerationperformances in a cryogenic environment in the neighborhood of 4.2 K andtheir lowest achieved temperatures are lower than those of theconventional regenerative materials. Thus with rare earth metaloxysulfide regenerative materials, it is easy to obtain small-sizedregenerators having a high refrigerating efficiency. The regenerativematerial according to the present invention can be used forsuperconducting magnets, refrigerators for cooling MRI, etc. Moreover,by selecting the kinds of rare earth elements and using a plurality ofrare earth elements, a desired magnetic phase transition temperature canbe obtained, and the range of the specific heat peak in the neighborhoodof the magnetic phase transition temperature can be extended. With theuse of spherical rare earth metal oxysulfide regenerative materialgranules, the transfer resistance of the refrigerant can be loweredwhile raising the packing density of the regenerative material.Furthermore, by reducing the surface roughness of the granules,generation of dust can be prevented, and the useful life of theregenerative material can be extended.

Regenerators are required to have a continuous range of specific heatfrom the higher temperature side to the lower temperature side and abroad range of distribution of specific heat in the neighborhood of thetarget refrigeration temperature. The former is a characteristic that isrequired for efficiently refrigerating down to the target temperature,and the latter is a characteristic for allowing the selection of atarget temperature from a wider range. As rare earth metal oxysulfideregenerative materials have small specific heats at 7 K and over, it isdesirable to arrange a regenerative material such as HoCu₂ on the highertemperature side of a rare earth metal oxysulfide regenerative material.As a single rare earth metal oxysulfide regenerative material canprovide only a narrow range of specific heat distribution, it isdesirable to provide, in layers, a rare earth metal oxysulfideregenerative material for lower temperatures and one for highertemperatures so that these materials as a whole can provide a continuousrange of specific heat. In particular, oxysulfides including Gd as themain component have insufficient specific heats at 6 K–7 K, it isdesirable to provide an oxysulfide including Tb as the main component onthe higher temperature side of the former. As for refrigerating down to4 K and under, it is desirable to provide an oxysulfide including Ho orDy as the main component on the lower temperature side of an oxysulfideincluding Gd as the main component.

Preparation of Rare Earth Metal Oxysulfide Regenerative Material

A rare earth metal oxysulfide is generated, for example, by placingpowder of a rare earth oxide in a reaction tube, and heating the tubewhile passing a gas containing sulfur atoms of oxidation number −2, suchas H₂S or CH3SH through the tube. Preferably, the reaction temperatureis 500–800° C., and more preferably, 600–700C. When the temperature isunder 500° C., it takes a long time to complete the reaction When thetemperature exceeds 800° C., the reaction will start to generate asulfide. Preferably, the reaction time is 1–9 hours, and morepreferably, 1–3 hours.

It is desirable to granulate the regenerative material. This is to makethe regenerative material more resistant to the compression at the timeof packing the material into a regenerator and to the heat impacts andvibrations during its use and prevent generation of dust. In particular,it is desirable to make the granules more spherical. Preferably, themean ratio of the largest dimension to the smallest dimension of thegranules (the mean aspect ratio) is 3 or under, and more preferably, 2or under, and much more preferably, in the neighborhood of 1, in otherwords, nearly true spheres. As rare earth metal oxysulfides are brittlerthan rare earth intermetallic compounds, when the mean aspect ratioexceeds 3, granules of a rare earth metal oxysulfide tend to break up.Moreover, when the mean aspect ratio exceeds 3, it will be difficult touniformly pack the granules into a regenerator.

Preferably, the mean particle size of the granules is 0.05–1 mm. Whenthe mean particle size is less than 0.05 mm, the packing density willget higher, and the He refrigerant can not pass sufficiently through theregenerator and the heat exchange efficiency will deteriorate. On theother hand, when the mean particle size exceeds 1 mm, the contact areawith the He refrigerant will get smaller and the heat exchangeefficiency will decrease. Hence, preferably, the mean particle size is0.05–1 mm, more preferably, 0.1–0.7 mm, and much more preferably,0.1–0.3 mm.

Preferably, the relative density of the granules of the regenerativematerial is 96% or over, more preferably, 98% or over, and much morepreferably, 99% or over, it is desirable to bring it close to thetheoretical density. When the relative density is less than 96%, themechanical strength of the granules will drop because of the presence ofa large number of open vacancies.

For enhancing the mechanical strength of the granules, preferably, themean grain size is 100 mμ or under, more preferably, 50 μm or under, andmuch more preferably, 10 μm or under and 1 μm or over. When the meangain size of the granules exceeds 100 μm, their mechanical strength willdecrease.

Irregularities on the surface of granules may serve as starting pointsof their failures. Hence, preferably, the surface roughness of thegranules is, for example, 10 μm or under on the maximum height (Rmax)standards defined in JIS B0601.

Preferably, the excess sulfur content in the granules is 10000 wtppm orunder, more preferably, 5000 wtppm or under, and most preferably, 2000wtppm or under. When a large amount of sulfur is present in thegranules, sintering inhibition will be caused and, in tun, themechanical strength of the granules will decrease. Control of the excesssulfur content can be done easily by, for example, controlling the anamount of H₂S gas flow at the time of sulfurization of a rare earthoxide.

The granules can be produced from a rare earth metal oxysulfide powderby a variety of methods. For example, the tumbling pelletizing, acombination of the extruding and the tumbling pelletizing, the fluidizedgranulating, the spray dry, or the template pressing may be used. It isdesirable to form the granules into spheres. After forming, the granulesare sorted to have an optimal particle size and/or an optimal aspectratio by sieving, shape classification, etc. A rare earth oxide powdermay be granulated in advance by any of the above-mentioned methods, thenthe sulfurization may be effected. The sulfurization conditions areidentical to those when an oxide powder material is used.

Formed bodies of a rare earth metal oxysulfide is to be sintered. Toprevent the rare earth metal oxysulfide from being oxidized, preferably,the sintering atmosphere is vacuum (10⁻³ torr or under) or an inert gassuch as argon or nitrogen. Preferably, the sintering temperature is1100–1600° C., and the sintering time is 1–10 hours.

If HIP treatment is given after sintering, the granules will become morecompact and have a larger mechanical strength. The sintering atmospherein HIP treatment is, for example, argon, and preferably, the treatmenttemperature is 1200–1500° C., and the pressure is 50–200 MPa

To keep the surface roughness of the granules at, for example, 10 μm orunder on the maximum height Rmax standards, preferably, the sinteredgranules are polished. For example, granules of the regenerativematerial and free abrasives are put into a processing vessel. If aprocessing fluid is needed, it will be added together with the media Thework is put in the vessel or the processing vessel to be moved so as tobe polished through the relative movement between the work and theabrasives or the media. For example, rotary barrel finishing,centrifugal fluidized barrel finishing, vibration barrel finishing, gyrofinishing, reciprocating finishing, or linear fluidized finishing may beused.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the heat capacities of rare earth metal oxysulfideregenerative materials, helium, and conventional regenerative materials.

FIG. 2 shows the heat capacities of Gd-rich Gd_(2−x)Tb_(x)O₂S rare earthmetal oxysulfide regenerative materials of the embodiments.

FIG. 3 shows the heat capacities of Tb-rich Gd_(2−x)Tb_(x)O₂S rare earthmetal oxysulfide regenerative materials.

FIG. 4 shows the heat capacities of Ho-Dy composite rare earth metaloxysulfide regenerative materials.

FIG. 5 shows the construction of a conventional regenerator (A) and thatof the regenerator (B) of the embodiment.

FIG. 6 shows the refrigerating capacity of a conventional regenerator(a) and those of the regenerators of the embodiments (b) and (c).

FIG. 7 shows the relative refrigerating capacities of a conventionalregenerator and a regenerator of an embodiment.

FIG. 8 shows the beat capacities of Gd_(x)Tb_(2−x)O₂S ceramics.

EMBODIMENTS

In the following, some embodiments and comparative examples will bedescribed Regenerative materials were packed into a regenerator under apacking pressure of 100 kPa, and the helium gas transfer resistance wasdetermined from the differential pressure between the top end and thebottom end of the regenerator. The mean aspect ratio was determined bytaking a microscopic photograph of the granules after sintering andmeasuring the ratio of the longest dimension to the shortest dimensionwith an image recognition device. The degree of dust generation wasdetermined by visual inspection of the regenerative material recoveredfrom the regenerator and finding the ratio of broken granules. Thecontent of the excess sulfur was determined by comparing the Gd contentobtained by a chemical analysis and the sulfur content obtained by acombustion analysis. Its unit is wtppm.

Preparation of Oxysulfides and their Heat Capacities

10 g of gadolinium oxide of which mean particle size determined withFisher method was 0.46 μm was filed in a quartz boat and the boat wasput into a quartz reaction tube. While H₂S gas was passed through thetube at a flow rate of 0.2 L/min, gadolinium oxide was made to react at650° C. for 2 hours. When the reaction products were measured with x-raydiffraction analysis, only the peak of gadolinium oxysulfide, Gd₂O₂S wasobserved. Hence the reaction yield based on the rare earth oxide was100%. The obtained Gd₂O₂S powder was formed into discs of 12 mm indiameter under a pressure of 30 MPa The discs were pressed by ahydrostatic press under a pressure of 200 MPa, then the discs weresubjected to atmospheric sintering in argon atmosphere at 1500° C. for 6hours to obtain the Gd₂O2S samples (embodiment 1).

The density of the sintered Gd₂O₂S of embodiment 1 was determined to be99.9% of the theoretical density with Archimedes' method. Its mean grainsize was calculated to be 3.2 μm from the following formula:d=1.56C/(MN)(wherein d: mean grain size; C: length of a line freely drawn on a highresolution image of SEM, etc.; N: number of crystal grains on the freelydrawn line; M: magnification of the image.)

The excess sulfur content of the sintered Gd₂O₂S of embodiment 1 wasfound to be 1000 wtppm by comparing the Gd content obtained by thechemical analysis and the suffer content determined with the combustionanalysis.

The heat capacity of the sintered Gd₂O₂S of embodiment 1 is shown inFIG. 1, and its magnetic phase transition temperature and its heatcapacity at that temperature are shown in Table 1. Besides the sinteredGd₂O₂S, FIG. 1 shows the heat capacities of Tb₂O₂S, Dy₂O₂S and Ho₂O₂S,and for reference, the heat capacity of helium (He-0.5 MPa) and heatcapacities of Pb, ErNi and HoCu₂ being the conventional regenerativematerials. The Gd₂O₂S regenerative material of embodiment 1 had itsmagnetic phase transition temperature in the neighborhood of 5 K, andits heat capacity at the magnetic phase transition temperature is 1.2J/cc·K; the Gd₂O₂S regenerative material had a heat capacity 3 to 5times as large as those of the conventional regenerative materials suchas HoCu₂ or ErNi in the neighborhood of the liquid helium temperature.

TABLE 1 Magnetic Phase Transition Temperatures Magnetic phase transitionHeat capacity/ Sample Composition temperature/K J/cc · K Embodiment 1Gd₂O₂S 5.2 1.2 Embodiment 2 Ho₂O₂S 2.2 1.25 Dy₂O₂S 4.6 1.0 Embodiment 3Gd_(1.8)Tb_(0.2)O₂S 4.8 0.84 Gd₁Tb₁O₂S 4.2 0.61 Tb_(1.8)Gd_(0.2)O₂S 5.31.3 Tb₂O₂S 6.3 1.7 Embodiment 4 Dy_(1.8)Ho_(0.2)O₂S 4.3 0.8Ho_(1.8)Dy_(0.2)O₂S 2.4 0.85 Embodiment 5 Gd_(1.8)Y_(0.2)O₂S 4.6 0.75Gd_(1.8)La_(0.2)O₂S 4.6 0.85 Gd_(1.8)Ce_(0.2)O₂S 4.7 0.74Gd_(1.8)Pr_(0.2)O₂S 4.7 0.69 Gd_(1.8)Nd_(0.2)O₂S 4.8 0.77Gd_(1.8)Sm_(0.2)O₂S 4.8 0.63 Gd_(1.8)Eu_(0.2)O₂S 4.9 0.76Gd_(1.8)Dy_(0.2)O₂S 4.9 0.82 Gd_(1.8)Ho_(0.2)O₂S 4.9 0.71Gd_(1.8)Er_(0.2)O₂S 5 0.81 Gd_(1.8)Tm_(0.2)O₂S 5 0.73Gd_(1.8)Yb_(0.2)O₂S 5.1 0.76 Gd_(1.8)Lu_(0.2)O₂S 5.2 0.8

The gadolinium oxide Gd₂O₃ used in embodiment 1 was used withoutsulfurization to prepare a sintered material under the same conditionsof embodiment 1 (comparative example 1). The magnetic phase transitiontemperature of this sample was in the neighborhood of 1 K, and its heatcapacity in the neighborhood of 4.2 K was extremely small.

Holmium oxide, of which mean particle size was 0.36 μm, and dysprosiumoxide, of which mean particle size was 0.6 μm, were subjectedsulfurization, forming, hydrostatic pressing and sintering in the samemanner as embodiment 1 to obtain a sintered Ho₂O₂S and a sintered Dy₂O₂S(embodiment 2). The heat capacities of the sintered materials obtainedare shown in FIG. 1, and their magnetic phase transition temperaturesand their heat capacities at those temperatures are shown in Table 1,respectively. They exhibited larger heat capacities in wide ranges oftemperatures in the neighborhood of the liquid helium temperature thanthose of HoCu₂ and ErNi.

Composite Oxysulfides

A mixture of the gadolinium oxide powder used in embodiment 1 andterbium oxide powder of which mean particle size was 0.69 μm wassubjected to sulfurization, forming, hydrostatic pressing and sinteringin the same manner as embodiment 1 to obtain sintered gadolinium-terbiumoxysulfides (Gd_(x)Tb_(2−x)O₂S) (embodiment 3). X-ray diffractionpatterns of the four kinds of sintered materials of which compositionswere varied within the range of 0≦x≦2(x=0.2, 1.0, 1.8, and 2.0)weremeasured. For x=2.0, only the peak of Tb₂O₂S was observed For x=0.2, 1.0and 1.8, peaks corresponding to solid dissolved Gd_(x)Tb_(2−x)O₂S whichdid not belong to Gd₂O₂S nor Tb₂O₂S were obtained.

The heat capacities of sintered Gd_(x)Tb_(2−x)O₂S of embodiment 3 areshown in FIG. 2, and their magnetic phase transition temperatures andtheir heat capacities at these temperatures are shown in Table 1. As thevalue of x decreases to 1.8, then to 1, the heat capacity at the time ofmagnetic phase transition decreases, whereas the magnetic phasetransition temperature shifts to the lower temperature side of that ofGd₂O₂S, the peak width of the specific heat increases, and the heatcapacity exceeds that of Gd₂O₂S at the liquid helium temperature. On theother hand, as shown in Table 1 and FIG. 3, as the compositionapproaches to that of Tb₂O₂S, the magnetic phase transition temperatureshifts to the higher temperature side of that of Gd₂O₂S.

Generally speaking, the magnetic interaction of rare earth magneticatoms in a crystal depends on the interatomic distance. If the crystalis perfect and the interatomic distances of magnetic atoms are identicalthe magnetic interaction can be expressed with a single parameter, andthe magnetic spin system of the crystal as a whole makes a sharp phasetransition. In that case, as is the case of embodiment 1, the peak ofthe specific heat becomes higher and sharper due to the phasetransition. On the other hand, as is the case of embodiment 3, when aplurality of rare earth elements are solid dissolved, the interatomicdistances of magnetic atoms will be varied locally, and the crystalfield will be disturbed locally, resulting in the loss of uniformity ofthe magnetic interaction of the entire crystal. As a result, themagnetic phase transition of the magnetic spins in the crystal will belocally disturbed, and the peak of the specific heat will be dispersedin a certain temperature range, resulting in an expansion of thespecific heat peak width. In a Ho system, this is accompanied by a shiftof the magnetic phase transition temperature to the higher temperatureside, and in Gd, Td and Dy systems, the magnetic phase transitiontemperature shifts to the lower temperature side.

The holmium oxide powder and the dysprosium oxide powder, which wereused in embodiment 2, were mixed and the mixture was subjected tosulfurization, forming, hydrostatic pressing and sintering in the samemanner as embodiment 1 to obtain sintered dysprosium-holmium compositeoxysulfides, Dy_(x)Ho_(2−x)O₂S (embodiment 4). The heat capacities ofthe sintered materials are shown in FIG. 4, and their magnetic phasetransition temperatures and their heat capacities at those temperaturesare shown in Table 1. Magnetic phase transition temperatures that arebetween those of Dy₂O₂S and Ho₂O₂S were obtained successfully bychanging the value of x, and the peak width of the specific heat weresuccessfully expanded wider than those of Dy₂O₂S and Ho₂O₂S.

Gadolinium oxide (90 mol %) and rare earth oxides (10 mol %) of Y, La,Ce, Pr, Nd, Sm, Eu, Dy, Er, Tm, Yb and Lu were treated in the samemanner as embodiment 3 to obtain sintered composite rare earth metaloxysulfides (embodiment 5). Their magnetic phase transition temperatures(Tc) and their heat capacities at those temperatures are shown inTable 1. With the use of various composite rare earth metal oxysulfides,a variety of magnetic phase transition temperatures can be obtained, andthe peak value of the specific heat at the magnetic phase transitiontemperature can be changed. The rare earth oxides used in embodiments 3and 5 were treated intact as oxides, without sulfurization, and treatedin the same manner as embodiment 3 to obtain sintered materials(comparative example 2). Their heat capacities in the neighborhood of4.2 K were extremely small.

Regenerative Material Granules

The Gd₂O₂S powder obtained in embodiment 1 was spherically formed by thetumbling pelletizing, and the obtained granules were sieved with twokinds of nylon meshes (mesh A (opening 308 μm) and mesh B (opening: 190μm)). The sieved granules were made to roll over a mirror-finished ironplate tilted at about 25°. The granules which rolled down were recoveredto make shape classification. The mean particle size and the mean aspectratio of 100 granules were 0.25 mm and 1.1, respectively. The meanparticle size and the mean aspect ratio of the Gd₂O₂S granules weremeasured on an image taken with a video high scope system.

The obtained Gd₂O₂S granules were filled in a crucible of alumina, andthe granules were put in a sintering furnace and subjected toatmospheric sintering. Then the furnace was fully vacuum-pumped, andargon gas was introduced to sinter the granules in argon atmosphere. Thesintering temperature was 1500° C. and the sintering time was 6 hours.Thus the desired Gd₂O₂S regenerative material was obtained. The densityof the Gd₂O₂S regenerative material measured by the pycnometer, was99.2% of the theoretical density. The mean grain size and the sulfurcontent were identical to those of embodiment 1.

Nylon media and alumina slurry of 10 wt % concentration were put in aprocessing vessel, then the Gd₂O₂S regenerative material was put intothe vessel to give surface treatment with rotary barrel finishing andobtain granules of the regenerative material (embodiment 6). When thetreatment time was 6 hours, the surface roughness of the granules was 1μm. The surface roughness was measured with a scanning tunnelingmicroscope (STM surface roughness meter). The obtained Gd₂O₂Sregenerative material was packed in the cooling section of a GMrefrigerator at a packing rate being close to the closest packing. ThenHe gas of which heat capacity was 25 J/K was subjected to the GMrefrigeration operation cycle continuously repeating for 500 hours undera mass flow rate of 3 g/sec and gas pressure of 16 atm. At that timepoint, the transfer resistance of He gas flowing through theregeneration section was measured No increase in the transfer resistancewas observed after the start of the operation. After 1000 hours ofcontinuous operation, the Gd₂O₂S regenerative material was taken out andexamined. There were no pulverized granules.

The following samples were prepared by sulfurization, forming,classification, sintering and polishing of granules of rare earth metaloxysulfides under conditions similar to those of embodiment 6. Theconditions of preparation were similar to those of embodiment 6 exceptsome points specified otherwise. The sample numbers were givenconsecutively, starting with embodiment 1 as sample 1.

Granules were made under the conditions similar to those of embodiment 6except that the inclined angle of the iron plate was changed. Then thegranules were sintered and polished (samples 2 and 3). The residualgranules of the shape classification of embodiment 6, of which aspectratios exceeded 3, were sintered and polished (sample 4). The helium gastransfer resistance and the degree of dust generation of each samplewere evaluated by the GM refrigeration operation cycle used inembodiment 6. The results are shown in Table 2. When the mean aspectratio was less than 3, good results were obtained just like embodiment6. When the mean aspect ratio exceeded 3, the helium gas transferresistance increased by 30 to 40% after 500 hours of continuousoperation. After 1000 hours of continuous operation, the ratio f finelybroken granules reached to 20–30%.

TABLE 2 Effects of the Aspect Ratio Classification Mean aspect Increaseof He gas Degree of dust Sample angle/° ratio transfer resistancegeneration 1 25 1.1 None No problem 2 30 1.3 None  No problem. 3 40 1.8None No problem 4 — 3.2 Increased by 30–40% 20–30% of granules after 500hrs were broken after operation 1000 hrs operation.

Sintering Conditions

Sintered granules were prepared under conditions similar to those ofembodiment 6 except that the sintering temperature or the sintering timewas changed to alter the grain size (samples 5 through 9). Influences ofthe changes in the grain diameter on the helium gas transfer resistanceand the degree of dust generation were evaluated with the GMrefrigeration operation cycle used in embodiment 6. The results areshown in Table 3. Good results were obtained for granules of which grainsizes were 100 μm or under, samples 1 and 5–7. However, for samples 8and 9, of which grain sizes exceeded 100 μm, the He gas transferresistance increased by 20 to 30% after 500 hours of continuousoperation, and the ratios of finely broken granules rose to 10–15% after1000 hours of continuous operation.

TABLE 3 Sintering Conditions and Durability Sintering Sintering Meangrain Increase of He gas Degree of dust Sample temp./° C. time/Hrsize/μm transfer resistance generation 1 1500 6 3.7 None No problem 51550 6 23 None No problem 6 1600 6 85 None No problem 7 1600 3 37 NoneNo problem 8 1650 6 110 20–30% after 500 10–15% of hours of operationgranules failed 9 1600 15 121 20–30% after 500 10–15% of hours ofoperation granules failed

Surface Roughness

The surface treatment time was varied to induce differences in thesurface roughness. The He gas transfer resistance and the degree of dustgeneration in relation to the surface roughness were evaluated with theGM refrigeration operation cycle used in embodiment 6. The results areshown in Table 4. When the surface roughness was 10 μm or under, as werethe cases of samples 1, 10 and 11, good results were obtained. When thesurface roughness exceeded 10 μm, as was the case of sample 12, the Hegas transfer resistance increased by 20–30% after 500 hours ofcontinuous operation, and the ratio of finely broken granules rose toabout 15–20% after 1000 hours of continuous operation.

TABLE 4 Surface Roughness and Durability Surface treatment Surfaceroughness/ Increase of He gas Degree of dust Sample time/Hr μm transferresistance generation  1 6 1 None No problem 10 4 5 None No problem 11 28 None No problem 12 0 12 20–30% after 500 15–20% of hours of operation.granules failed

Influences of Excess Sulfur

The gas flow rate for sulfurization was varied to prepare various Gd₂O₂Spowders. These powders were granuled and sintered in the same manner asembodiment 6. The content of excess sulfur influences the relativedensity as well as the He gas transfer resistance and the degree of dustgeneration. Hence the He gas transfer resistance and the degree of dustgeneration were evaluated with the GM refrigeration operation cycle usedin embodiment 6. The results are shown in Table 5. When the relativedensity of the granules was 96% or over, as were the cases of samples 1,13 and 14, good results were obtained. When the relative density of thegranules was less than 96%, as was the case of sample 15, the increaseof He gas transfer resistance rose to 15–20% after 500 hours ofcontinuous operation, and as for the state of breakage of granules after1000 hours of continuous operation, the ratio of finely broken granulesrose to about 5–10%. Preferably, to keep the relative density at 96% orover, the excess sulfur content is 10,000 wtppm or under. Morepreferably, to keep the relative density at 98% or over, the excesssulfur content is 5000 wtppm or under. Most preferably, to keep therelative density at 99% or over, the excess sulfur content is 1000 wtppmor under.

TABLE 5 Effects of Excess Sulfur H₂S gas flow Sulfur content/ RelativeIncrease of He gas Degree of dust Sample rate (L/min) wtppm density/%transfer resistance generation 1 0.2 1000 99.2 None No problem 13 1 500098.3 None No problem 14 1.25 7000 97.6 None No problem 15 2.5 12500 95.115–20% after 500 5–10% of hours of operation granules failed

Refrigerating Capacity

The refrigeration characteristics of the Gd₂O₂S regenerative materialprepared in embodiment 6 and the Gd_(1.8)Tb_(0.2)O₂S regenerativematerial prepared by a method similar to that of embodiment 6 wereexamined with a regenerative type pulse-tube refrigerator of which powerconsumption was 3.3 kW. Two stages of regenerators were provided in therefrigerator. Pb was used for the regenerator of the first stage of thehigher temperature side, and regenerative materials were packed in theregenerator of the second stage. FIG. 5(A) shows the construction of aregenerator of the second stage in a conventional case. The regeneratorwas packed with Pb, ErNi and HoCu₂ in the descending order oftemperature, and their volume ratio was 2:1:1. The refrigerationcharacteristics of the conventional case are shown in FIG. 6( a). Theoutput of this refrigerator at 4.2 K was about 165 mW and the lowestachieved temperature with no application was about 2.9 K.

On the other hand, the lower-temperature side 25 volume % of the HoCu₂regenerative material of this regenerator was substituted with theGd₂O₂S regenerative material or the Gd_(1.8)Tb_(0.2)O₂S regenerativematerial to examine the resulting refrigeration characteristics. Theconstruction of the regenerator of the embodiment is shown in FIG. 5(B).The refrigeration characteristics of the Gd₂O₂S regenerative materialare shown in FIG. 6( b), and the refrigeration characteristics of theGd_(1.8)Tb_(0.2)O₂S regenerative material are shown in FIG. 6( c). Whenthe Gd₂O₂S regenerative material was used, the output at 4.2 K was about300 mW, and the lowest achieved temperature with no application wasabout 2.7 K. When the Gd_(1.8)Tb_(0.2)O₂S regenerative material wasused, the output at 4.2 K was about 340 mW, and the lowest achievedtemperature with no application was about 2.65 K.

The relative refrigerating capacities of regenerators using Gd₂O₂S andGd_(1.8)Tb_(0.2)O₂S regenerative materials of FIGS. 6( b) and (c) areshown in FIG. 7, with the refrigerating capacity of the conventionalregenerator being set at 1. The refrigerating capacity, at 4.2 K, of theregenerator packed with the Gd₂O₂S regenerative material (dashed line a)was about twice that of the conventional regenerator, and with the dropin temperature, the factor of the regenerating capacity increased andreached to 4 times at 3 K. The refrigerating capacity of theGd_(1.8)Tb_(0.2)O₂S regenerative material (continuous line b) was twiceor more at 4.2 K, and with the drop in temperature, the factor of therefrigerating capacity increased and reached to 4.5 times at 3 K.

Influences of the Mean Particle Size of Granules on RefrigeratingCapacity

The size of the opening of the meshes used for sieving was varied toprepare regenerative material granules of varied mean particle sizes.Other conditions were similar to those of embodiment 6. Therefrigeration characteristics of the prepared granules were evaluatedsimilarly. The results are shown in Table 6. When the mean particle sizeof the granules was 0.05 mm or over and 1 mm or under, as were the casesof samples 16 through 18, high outputs were obtained at 4.2 K. When theparticle size of the granules were off this range, as were the cases ofsamples 19 and 20, the outputs at 4.2 K decreased. Accordingly,preferably, the mean particle size of the granules is 0.05 mm or overand 1 mm or under, and more preferably, 0.1–0.7 mm, and more preferably,0.1–0.3 mm, and most preferably, 0.2–0.3 mm.

TABLE 6 Influences of the Mean Particle Size Mean particle size Sampleof granules/mm Output at 4.2 K/mW 16 0.25 300 17 0.77 290 18 0.071 28519 1.1 200 20 0.045 185

Comparative Example 3

Rare Earth Oxide Regenerative Materials

The refrigeration characteristics of Gd₂O₃ granules, which were preparedunder forming, classification and sintering conditions similar to thoseof embodiment 6, were evaluated similarly. The output at 4.2 K was about100 mW, and the lowest achieved temperature with no application wasabout 3.5 K. The results were inferior to those of the conventionalexample (HoCu₂) in both terms, namely, the output and the lowestachieved temperature of the regenerator.

Supplement

As for the volume ratio of the HoCu₂ regenerative material and the rareearth metal oxysulfide regenerative material, preferably, HoCu₂ is20–80% and the rare earth metal oxysulfide is 80–20%. When a Tb-basedoxysulfide is arranged on the higher temperature side of a Gd-basedoxysulfide, it is preferred that HoCu₂ is arranged on the highertemperature side of the latter to secure a refrigerating capacity up to7 K. When Gd_(0.1)Tb_(1.9)O₂S was prepared as an oxysulfide including Tbas the main component, a regenerative material was obtained, of whichpeak of specific heat shifted a little to the lower temperature sidethan Tb₂O₂S but other characteristics were similar to those of Tb₂O₂S.

Best Embodiment (Addition of Toughenings)

In the following, some embodiments will be indicated, to which anadditive such as ZrO₂ was added to a rare earth metal oxysulfide toenhance the strength and the durability. To distinguish theseembodiments from the above-mentioned embodiments to which no additivewas added, the former embodiments were designated as embodiments 11through 28. The rare earth metal oxysulfides to which no additive wasadded were designated as examples 1 through 12. The related tables weredesignated as tables 11 through 21.

The addition of an additive did not produce much difference in themagnetic phase transition temperatures of the rare earth metaloxysulfide regenerative materials in comparison with those of thematerials without any additive. When an additive is less than 0.05 wt %,the durability in case of long hours of operation of the refrigeratorposes a problem. When the addition exceeds 30 wt %, the specific heat ofthe regenerative material decreases.

Besides the R₂O₂S phase, when an additive is added, a second phase whichincludes the additive and has a composition differing from that of themain phase will be formed. As a result, the crystal grain growth of themain phase will be restrained, and the strength will increase due to theinclusion of the second phase of relatively high strength. Additives maybe regarded as toughenings to the rare earth metal oxysulfide ceramicsmicrostructure. Preferable additives include Al₂O₃, ZrO₂, mullite,Si₃N₄, Sialon, TiN AlN, BN, SiC and TiC; in particular, ZrO₂, Si₃N₄,Sialon, TiN, AlN, BN, SiC and TiC are preferable. Besides them, oxidesof Mg, Ca, Sr and Ba, and oxides of transition metal elements of atomicnumbers 22 (Ti)–31 (Ga) and 72 (Hf) are preferable as additives. In thefollowing, for simplicity, these oxides are referred to as alkalineearth metal oxides and transition metal oxides.

When any of Al₂O₃, ZrO₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiC, TiC,alkane earth metal oxides and transition metal oxides is added, besidesthe R₂O₂S phase as the main phase, a second phase will be formed, andwith it, the crystal grain growth of the main phase will be restrained,and the strength of the rare earth metal oxysulfide regenerativematerial will increase. Accordingly, even if the refrigerator isoperated for long hours, the regenerative material granules will notbreak down or the sealing part or the like of the refrigerator will notbe damaged.

To produce a rare earth metal oxysulfide regenerative material with anadditive, the additive or the precursor thereof is added at aconcentration of 0.05–30 wt % to, for example, a powder having a generalformula R₂O₂S, and the mixture is formed into, for example, granules. Orthe additive or the precursor thereof in the form of powder is added toa raw material, rare earth oxide powder. A gas containing sulfur atom ofoxidation number −2 such as H₂S or CH₃SH is flowed into the raw materialunder heating to make them react to produce an oxysulfide. Then theoxysulfide is formed into, for example, granules. Next, these granulesare sintered at, for example, 1400–1600° C. for 1–10 hours. The relativedensity of the resulting rare earth metal oxysulfide regenerativematerial was 98% or over, and the mean grain size was 20 μm or under.

For example, an additive or its precursor is added to a rare earth oxidepowder available in the market, and the mixture is mixed in a ball millor the like. After the addition, the mixed powder is sintered at about800–1100° C., and the sintered mixed powder is filled in a reaction tubeof quartz or the like. Then a gas containing H₂S is flowed through thepowder to make sulfurization. As a result, the desired rare earth metaloxysulfide powder is obtained. A rare earth oxide powder may be made toundergo the sulfurization reaction, and after that, an additive or itsprecursor may be mixed with the powder.

The packing of the regenerative material into the regenerator was doneat a packing pressure of 100 KPa as before. As for the distinctionbetween the R₂O₂S phase as the main phase and the second phase differingfrom the main phase in the microstructures of the sintered material, thekinds of phases were determined by X-ray diffraction, and thedistribution of phases was examined with a metallograph. The ratio ofthe main phase and the second phase was determined by grinding andpolishing the surface of the sintered material, photographing thespecimen surface through a metallograph, measuring the ratio with animage recognition device, and converting the area ratio to the volumeratio. The mean aspect ratio of the granules was determined byphotographing the sintered granules through a microscope and measuringthe ratio of the longer dimension and the shorter dimension with animage recognition device. The state of breakage of the granules wasdetermined by visually examining the regenerative material recoveredfrom the regenerator and finding the ratio of broken granules.

EXAMPLE 1

A rare earth metal oxysulfide was prepared by the same manner asembodiment 1 (the programming rate for temperature up was 200° C./h).This sample was designated as example 1.

Embodiment 11 Addition of Zirconia

The gadolinium oxide used in example 1 and partially stabilized zirconia(3Y—ZrO₂, 3 mol % Y₂O₃—97 mol % ZrO₂; the same was used in thefollowing) were put in a ball mill and mixed for 24 hours with ethanolas a solvent. The resulted slurry was dried and calcined (900° C.×3hours). The product was made to react with hydrogen sulfide gas toprepare Gd₂O₂S ceramics containing Zr (Zr-doped Gd₂O₂S) in the samemanner as example 1 (hydrostatic pressing at 200 MPa, then atmosphericsintering in argon atmosphere at 1500° C. for 6 hours). The density ofthe resulting Zr-doped Gd₂O₂S was determined to be 99.9% of thetheoretical density by Archimedes' method, and the mean grain size was1.1–1.5 μm. Table 11 shows the heat capacity at the magnetic phasetransition temperature (the temperature of the highest peak of the heatcapacity) corresponding to the amount of addition of ZrO₂, and the heatcapacity at 4.2K. As can be seen in Table 11, with the addition of ZrO₂,the heat capacity at the magnetic phase transition temperaturedecreases, but when the addition is 30 wt % or under, the heat capacityat 4.2K is 0.3 J/cc·K or over. When the beat capacity exceeds 0.3J/cc·K, there will be no large influences on the cooling characteristicsof the regenerator.

These samples were ground and polished, and the sample faces weresubjected to X-ray diffraction to determine the kinds of phases. Thedistribution of phases was examined under a metallograph. The presenceof a phase differing from the main phase was confirmed. It was ZrO₂phase evenly dispersed through the main phase. Image analysis alsorevealed that with the increase in the addition of ZrO₂, the ratio ofZrO₂ phase increases. This phase is considered to be ZrO₂ whichprecipitated because it could not be solid dissolved in the main phase.The details, however, are not clear yet. Naturally, the cause of thedecrease in the heat capacity at 4.2 K with the increase in the additionof ZrO₂ is the increase of the ZrO₂ phase. These observations were alsotrue when the kinds of the rare earth elements were changed. The reasonis that they are the properties concerning ceramics microstructures andheat capacity when the second phase such as ZrO₂ is present throughR₂O₂S phase as the main phase. These properties do not basically dependon the kinds of rare earth elements when the additive is the same.

TABLE 11 Addition of Partially Stabilized ZrO₂ Heat capacity at themagnetic Amount of phase transition temp./ Heat capacity at 4.2 K/Sample additive/wt % J/cc · K J/cc · K Example 1 Non-dope 1.2 0.50Embodiment 11 0.1 1.2 0.50 ″ 0.5 1.1 0.50 ″ 1 1.0 0.50 ″ 10 0.89 0.48 ″20 0.66 0.38 ″ 30 0.49 0.32 ″ 40 0.38 0.23Embodiment 12 Addition of Alumina

Gd₂O₂S ceramics containing Al₂O₃ (Al-doped Gd₂O₂S) were prepared withAl₂O₃ instead of partially stabilized zirconia (3Y—ZrO₂). The otherconditions were similar to those of embodiment 11. The density of theresulting Al-doped Gd₂O₂S was determined to be 99.9% of the theoreticaldensity by Archimedes' method, and the mean grain size was 1.1–1.5 μm.Table 12 shows the heat capacity at the magnetic phase transitiontemperature (the temperature of the highest peak of heat capacity)corresponding to the amount of addition of Al₂O₃, and the heat capacityat 4.2K. As can be seen in Table 12, with the addition of Al₂O₃, theheat capacity at the magnetic phase transition temperature decreases,but when the addition is 30 wt % or under, the heat capacity at 4.2K is0.3 J/cc·K or over.

TABLE 12 Addition of Al₂O₃ Heat capacity at the magnetic Amount of phasetransition temp./ Heat capacity at 4.2 K/ Sample additive/wt % J/cc · KJ/cc · K Example 1 Non-dope 1.2 0.50 Embodiment 12 0.1 1.2 0.50 ″ 0.51.1 0.50 ″ 1 1.0 0.50 ″ 10 0.87 0.47 ″ 20 0.64 0.37 ″ 30 0.48 0.33 ″ 400.39 0.24Embodiment 13 Addition of Mullite

Gd₂O₂S ceramic containing mullite were prepared with mullite,3Al₂O₃−2SiO₂ instead of partially stabilized zirconia (3Y—ZrO₂). Theother conditions were similar to those of embodiment 11. Moreover,Gd₂O₂S ceramics were prepared with a non-oxide such as Si₃N₄, Sialon,TiN, AlN, BN, Sic, or TiC instead of partially stabilized zirconia(3Y—ZrO₂). The conditions were similar to those of embodiment 11 exceptno calcination was made before sulfurization. When the additive waschanged to mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiC or TiC, the resultswere comparable to those of embodiments 11 and 12 when the amount ofaddition was the same.

Embodiment 14 Addition of CaO

Gd₂O₂S ceramic with the addition of CaO (Ca-doped Gd₂O₂S) were preparedwith CaO instead of partially stabilized zirconia (3Y—ZrO₂). The otherconditions were similar to those of embodiment 11. The density of theresulting Ca-doped Gd₂O₂S was determined to be 99.9% of the theoreticaldensity by Archimedes' method; and the mean grain size was 1.9–2.1 μm.Table 13 shows the heart capacity at the magnetic phase transitiontemperature (the temperature of the highest peak of heat capacity)corresponding to the amount of CaO addition, and the heat capacity at4.2. K. As can be seen in Table 13, with the CaO addition, the heatcapacity at the magnetic phase transition temperature decreases, butwhen the addition is 30 wt % or under, the heat capacity at a desiredtemperature higher than 10 K is 0.3 J/cc·K or over. These samples wereground and polished, and the sample faces were subjected to X-raydiffraction to determine the kinds of phases. The distribution of phaseswas examined under a metallograph. The presence of a phase differingfrom the main phase and containing CaO was confirmed. It was evenlydispersed through the main phase. With the increase in the addition ofCaO, the ratio of the phase containing CaO increased. This phase isconsidered to be formed by the deposit of CaO which could not be soliddissolved in the main phase.

TABLE 13 Addition of CaO Heat capacity at the magnetic Amount of phasetransition temp./ Heat capacity at 4.2 K/ Sample additive/wt % J/cc · KJ/cc · K Example 1 Non-dope 1.2 0.50 Embodiment 14 0.07 1.2 0.50 ″ 0.71.1 0.50 ″ 1.4 1.0 0.49 ″ 14 0.88 0.47 ″ 28 0.67 0.37 ″ 42 0.47 0.27Embodiment 15

Gd₂O₂S ceramics with the addition of MgO (Mg-doped Gd₂O₂S) were preparedwith MgO instead of CaO. The other conditions were similar to those ofembodiment 14. The density of the resulting Mg-doped Gd₂O₂S wasdetermined to be 99.9% of the theoretical density by Archimedes' method,and the mean gain size was 1.9–2.2, μm. Like the Ca-doped Gd₂O₂S, theheat capacity at 4.2 K was 0.3 J/cc·K or over when the amount ofaddition of MgO was 30 wt % or under.

Embodiment 16

SrO or BaO were used instead of CaO, and other conditions of preparationwere similar to those of embodiment 14. The results of Sr-doped Gd₂O₂Sand Ba-doped Gd₂O₂S were comparable to those of embodiments 14 and 15.

Embodiment 17 Addition of Cr₂O₃

Cr₂O₃ was used instead of partially stabilized zirconia to prepareGd₂O₂S ceramics (Cr-doped Gd₂O₂S). The other conditions were similar tothose of embodiment 11. The density of the resulting Cr-doped Gd₂O₂S wasdetermined to be 99.9% of the theoretical density by Archimedes' method,and the mean grain size was 2.0–2.3 μm. Table 14 shows the heat capacityat the magnetic phase transition temperature (the temperature of thehighest peak of heat capacity) corresponding to the amount of additionof Cr₂O₃, and the heat capacity at 4.2K. As can be seen in Table 14,with the addition of Cr₂O₃, the heat capacity at the magnetic phasetransition temperature decreases, but when the addition of Cr₂O₃ is 30wt % or under, the heat capacity at 4.2 K is 0.3 J/cc·K or over. Theseobservations were also true for samples to which any of transition metaloxides other than Cr₂O₃ was added. These samples were ground andpolished, and the sample faces were subjected to X-ray diffraction andwere examined under a metallograph to find the distribution of the mainphase and the second phase. It was found that the second phase differingfrom the main phase containing Cr₂O₃ was evenly dispersed through themain phase.

TABLE 14 Addition of Cr₂O₃ Heat capacity at the magnetic Amount of phasetransition temp./ Heat capacity at 4.2 K/ Sample additive/wt % J/cc · KJ/cc · K Example 1 Non-dope 1.2 0.50 Embodiment 17 0.07 1.2 0.50 ″ 0.71.1 0.50 ″ 1.4 1.0 0.49 ″ 14 0.85 0.46 ″ 28 0.68 0.38 ″ 42 0.46 0.28Embodiment 18

Gd₂O₂S ceramics containing MnO (Mn-doped Gd₂O₂S) were prepared with MnOinstead of Cr₂O₃. The other conditions were similar to those ofembodiment 17. The density of the resulting Mn-doped Gd₂O₂S wasdetermined to be 99.9% of the theoretical density by Archimedes' method,and the mean grain size was 2.0–2.3 μm. Like Cr-doped Gd₂O₂S, when theaddition of MnO was 30 wt % or under, the heat capacity at 4.2 K was 0.3J/cc·K or over.

Embodiment 19

A Gd₂O₂S ceramics regenerative material was prepared by using atransition metal oxide other than Cr₂O₃ and MnO as the additive. Otherconditions were similar to those of embodiment 17. The results of thisregenerative material were comparable to those of embodiments 17 and 18.

Embodiment 20 Zirconia Addition to Gd—Tb Composite Oxysulfides

Partially stabilized zirconia (3Y—ZrO₂) was added to a mixture ofterbium oxide of which mean particle size was 0.69 μm and the gadoliniumoxide used in embodiment 11. They were subjected to sulfurization,forming, hydrostatic pressing and sintering in a manner similar to thatof example 1 to prepare gadolinium-terbium oxysulfide ceramicscontaining partially stabilized zirconia (3Y—ZrO₂) (Zr-dopedGd_(x)Tb_(2−x)O₂S). FIG. 8 shows the heat capacities of GdxTb_(2−x)O₂S.Tables 15 through 17 show the amount of addition of ZrO₂ and the heatcapacity at the desired temperature for various values of x. Examples 2through 7 are Gd_(x)Tb_(2−x)O₂S ceramics with no addition of ZrO₂. Fromtables 15 through 17, it can be seen that a heat capacity of 0.3 J/cc·Kor over is secured for a relatively wide range of temperatures of 10 Kor under. For x≧1, Zr-doped Gd_(x)Tb_(2−x)O₂S can be used as aregenerative material for the neighborhood of 4.2 K. For x<0.1, it canbe used as a regenerative material for the neighborhood of 6 K–7 K. Evenwhen ZrO₂ is added up to 30 wt %, the heat capacity at any desiredtemperatures of 10 K or under never fall below 0.3 J/cc·K. Similarresults were obtained when Gd or Tb was substituted with another rareearth element such as Dy or Ho.

TABLE 15 Addition of ZrO₂ Amount of additive/ Heat capacity at Sample wt% Value of x 4.2 K/J/cc · K Example 2 Non-dope 1.8 0.55 Embodiment 200.1 1.8 0.55 ″ 0.5 1.8 0.55 ″ 1 1.8 0.55 ″ 10 1.8 0.51 ″ 20 1.8 0.41 ″30 1.8 0.36 ″ 40 1.8 0.26 Example 3 Non-dope 1 0.59 Embodiment 20 0.1 10.59 ″ 0.5 1 0.59 ″ 1 1 0.59 ″ 10 1 0.55 ″ 20 1 0.43 ″ 30 1 0.39 ″ 40 10.28

TABLE 16 Addition of ZrO₂ Amount of additive/ Heat capacity at Sample wt% Value of x 5.0 K/J/cc · K Example 4 Non-dope 0.2 0.71 Embodiment 200.1 0.2 0.71 ″ 0.5 0.2 0.71 ″ 1 0.2 0.71 ″ 10 0.2 0.68 ″ 20 0.2 0.5  ″30 0.2 0.41 ″ 40 0.2 0.29

TABLE 17 Addition of ZrO₂ Amount of additive/ Heat capacity at Sample wt% Value of x 6.0 K/J/cc · K Example 5 Non-dope 0.1 1 Embodiment 20 0.10.1 1 ″ 0.5 0.1 1 ″ 1 0.1 0.93 ″ 10 0.1 0.84 ″ 20 0.1 0.64 ″ 30 0.1 0.56″ 40 0.1 0.28 Example 6 Non-dope 0.05 1.2 Embodiment 20 0.1 0.05 1.2 ″0.5 0.05 1.2 ″ 1 0.05 1.1 ″ 10 0.05 0.92 ″ 20 0.05 0.73 ″ 30 0.05 0.51 ″40 0.05 0.29 Example 7 Non-dope 0 0.88 Embodiment 20 0.1 0 0.88 ″ 0.5 00.88 ″ 1 0 0.88 ″ 10 0 0.72 ″ 20 0 0.51 ″ 30 0 0.42 ″ 40 0 0.28Embodiment 21

Regenerative material ceramics were prepared with Al₂O₃, mullite,non-oxides such as Si₃N₄, Sialon, TiN, AlN, BN, or TiC instead ofpartially stabilized zirconia (3Y—ZrO₂). Other conditions were similarto those of embodiment 20. The results obtained from Gd_(x)Tb_(2−x)O₂Sceramics with different kinds of additives were comparable to those ofembodiment 20.

Embodiment 22

Gadolinium-terbium oxysulfide ceramics (Ca-doped Gd_(x)Tb_(2−x)O₂S) wereprepared with any of alkaline earth metal oxides (MgO, CaO, SrO, BaO) bysubstituting partially stabilized zirconia (3Y—ZrO₂). Other conditionswere similar to those of embodiment 20. The heat capacitycharacteristics of the ceramics showed tendencies similar to those ofZr-doped Gd_(x)Tb_(2−x)O₂S when the alkaline earth metal oxide was 30 wt% or under.

Embodiment 23

Gadolinium-terbium oxysulfide ceramics (Ca-doped Gd_(x)Tb_(2−x)O₂S)regenerative materials were prepared with any of transition metal oxides(oxides of elements of which atomic numbers are 22 (Ti) to 31 (Ga) and72 (Hf)) by substituting partially stabilized zirconia Other conditionswere similar to those of embodiment 20. The results obtained from thesematerials were similar to those of Zr-doped Gd_(x)Tb_(2−x)O₂S when thetransition metal oxide was 30 wt % or under.

Embodiment 24 Durability Under Continuous Operation

The Zr-doped Gd₂O₂S powders (after sulfurization and before sintering)of embodiment 11 were spherically formed by the tumbling pelletizing,and the obtained granules were sieved with two kinds of filter nets(mesh A (opening: 597 μm) and mesh B (opening: 435 μm)). The sievedgranules were made to roll over a mirror-finished iron plate tilted atabout 25°. The granules which rolled down were recovered to make shapeclassification. The mean particle size of 100 granules was 0.5 mm. Themean particle size of the Zr-doped Gd₂O₂S granules was measured on animage taken with a video high scope system.

The obtained Zr-doped Gd₂O₂S granules were filled in a crucible ofalumina, and the granules were subjected to atmospheric sintering inargon atmosphere in the manner described above. The sinteringtemperature was 1500° C. and the sintering time was 6 hours. ThusZr-doped Gd₂O₂S regenerative materials of which mean particle size was0.4 mm and mean aspect ratio was 1.1 were obtained. The mean particlesize and the mean aspect ratio of the Zr-doped Gd₂O₂S regenerativematerials were measured on a video high scope image. The density of theGd₂O₂S regenerative materials measured by the pycnometer, was 99.9% ofthe theoretical density, and the mean grain size was 1.1–1.5 μm.

In a way similar to that of embodiment 6, Zr-doped Gd₂O₂S regenerativematerials were surface-treated by rotary barrel finishing. The obtainedZr-doped Gd₂O₂S regenerative materials were packed in a regenerator of aGM refrigerator, and the state of breakage of granules after 1500 hours,2500 hours and 10000 hours of continuous operation in a way similar tothat of embodiment 6. The results are shown in Table 18. Example 8 areGd₂O₂S ceramics granules to which no ZrO₂ was added. When ZrO₂ added by0.05 wt % or over, no problem occurred even after 10,000 hours. When theaddition was 0.01 wt %, no significant differences were observed in thedurability. This was attributed to lack of the strengthening phase (ZrO₂phase). Similar tendencies were also observed when Gd was substitutedwith another rare earth element such as Dy or Ho.

TABLE 18 Addition of ZrO₂ Amount of additives/ After 1500 hrs After10000 hrs Sample wt % operation operation Example 8 Non-dope ca. 5% ofgranules failed. — Embodiment 24 0.01 ca. 5% of granules failed. — ″0.05 No problem No problem ″ 0.5 No problem No problem ″ 1 No problem Noproblem ″ 10 No problem No problem ″ 20 No problem No problem ″ 30 Noproblem No problem ″ 40 No problem No problemEmbodiment 25

The Gd₂O₂S powders (after sulfurization and before sintering) obtainedin embodiment 12 (Al₂O₃) and embodiment 13 (mullite) were used, withother conditions being similar to those of embodiment 24. The granuleswere subjected continuously to GM refrigeration operation cycle, and thestate of breakage of granules was observed after 1500 hours, 2500 hoursand 10000 hours. Like the Zr-doped Gd₂O₂S ceramics granules, the ceramicgranules with addition of the additive by 0.05 wt % or over showed noproblem even after 10000 hours of continuous operation of the GMrefrigerator. The ceramic granules with addition of the additive by lessthan 0.05 wt %, however, had finely broken granules after 1500 hours ofcontinuous operation.

Embodiment 26

The Gd₂O₂S powders (to which alkaline earth oxides were added) describedin embodiments 14 through 16 were used, with other condition beingidentical to those of embodiment 24. The granules were subjectedcontinuously to GM refrigeration operation cycle, and the state ofbreakage of granules was observed after 1500 hours, 2500 hours and 10000hours. The results of addition of CaO are shown in Table 19. The ceramicgranules to which CaO was added by 0.05 wt % or over had finely brokengranules after 10000 hours of continuous operation. On the other hand,granules to which CaO was added by less than 0.05 wt % had finely brokengranules after 1500 hours of continuous operation Similar tendencieswere observed when CaO was substituted with another alkaline earth metaloxide such as MgO, SrO or BaO, or when Gd was substituted with anotherrare earth element such as Dy or Ho.

TABLE 19 Addition of CaO Amount of additives/ After 1500 hrs After 2500hrs After 10000 hrs Sample wt % operation operation operation Example 8Non-dope ca. 5% failed. — Embodiment 26 0.01 ca. 5% failed. — ″ 0.07 Noproblem No problem ca. 20% failed. ″ 0.7 No problem No problem ca. 15%failed. ″ 1.4 No problem No problem ca. 15% failed. ″ 14 No problem Noproblem ca. 10% failed. ″ 28 No problem No problem ca. 10% failed. ″ 42No problem No problem ca. 10% failed.Embodiment 27 Addition of Transition Metal Oxides

The Gd₂O₂S powders to which transition metal oxides were added inembodiments 17 through 19 were used, with other condition beingidentical to those of embodiment 24. The granules were subjectedcontinuously to GM refrigeration operation cycle, and the state ofbreakage of granules was observed after 1500 hours, 2500 hours and 10000hours. The results were similar to those of granules to which alkalineearth metal oxides were added. The ceramics regenerative materials towhich the transition metal oxide was added by 0.05 wt % or over endured2500 hours of continuous operation, but they could not endure 10000hours of continuous operation. The ceramics regenerative materials towhich the transition metal oxide was added by less than 0.05 wt % hadfinely broken granules after 1500 hours of continuous operation. Similartendencies were observed when Gd was substituted with another rare earthelement such as Dy or Ho.

Embodiment 28 Durability of Gd—Tb Composite Oxysulfides

Like the samples described in embodiment 20, the value of x was varied,with other conditions being similar to those of embodiment 24, toprepare ceramics granules to which ZrO₂ was added. The state of breakageof granules in relation to differences in the value of x ofGd_(x)Tb_(2−x)O₂S was evaluated with the GM refrigeration operationcycle test used in embodiment 24. The results are shown in Table 20.Examples 9 through 12 are Gd_(x)Tb_(2−x)O₂S ceramics granules to whichno ZrO₂ was added. Even when the value of x of Gd_(x)Tb_(2−x)O₂S wasvaried, the ceramics granules to which ZrO₂ was added by 0.05 wt % orover posed no problem even after 10000 hours of continuous operation ofthe GM refrigerator. On the other hand, the ceramics granules to whichZrO₂ was added by less than 0.05 wt % had finely broken granules after1500 hours of continuous operation.

TABLE 20 Addition of ZrO₂ Amount of additive/ After 1500 hrs After 10000hrs Sample wt % Value f x operation of operation Example 9 Non-dope 1.8ca. 5% failed — Embodiment 28 0.01 1.8 ca. 5% failed — ″ 0.05 1.8 Noproblem No problem ″ 0.1 1.8 No problem No problem ″ 1 1.8 No problem Noproblem ″ 10 1.8 No problem No problem ″ 30 1.8 No problem No problemExample 10 Non-dope 1 ca. 5% failed — Embodiment 28 0.05 1 No problem Noproblem ″ 0.1 1 No problem No problem ″ 1 1 No problem No problem ″ 10 1No problem No problem ″ 30 1 No problem No problem Example 11 Non-dope0.2 ca. 5% failed — Embodiment 28 0.05 0.2 No problem No problem ″ 0.10.2 No problem No problem ″ 1 0.2 No problem No problem ″ 10 0.2 Noproblem No problem ″ 30 0.2 No problem No problem Example 12 Non-dope 0ca. 5% failed — Embodiment 28 0.05 0 No problem No problem ″ 0.1 0 Noproblem No problem ″ 1 0 No problem No problem ″ 10 0 No problem Noproblem ″ 30 0 No problem No problemConclusions Concerning Additives

As described above, what excel most in the durability under continuousoperation of refrigerators and the heat capacity are ceramics granulesto which any of Al₂O₃, ZrO₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiCand Tic was added by 0.05–30 wt % in total followed by ceramics granulesto which any of alkaline earth metals (oxides of Mg, Ca, Sr and Ba) andtransition metal oxides (oxides of elements of which atomic numbers arefrom 22 (Ti) through 31 (Ga) and 72 (Hf)) was added by 0.05 wt % –30 wt%.

Refrigerating Capacities

The refrigeration characteristics of the regenerative materials, theZr-doped Gd₂O₂S to which ZrO₂ was added by 10 wt % (embodiment 24), theAl-doped Gd₂O₂S to which Al₂O₃ was added by 10 wt % (embodiment 25), andthe Zr-doped Gd_(1.8)Tb_(0.2)O₂S to which ZrO₂ was added by 10 wt %(embodiment 28), the Gd₂O₂S to which no additive was added (example 8),and the Gd_(1.8)Tb_(0.2)O₂S to which no additive was added (example 9)were examined with a two-stage type GM refrigerator of which powerconsumption was 3.4 kW. In the conventional refrigerator wherein Pb wasused for the regenerator of the first stage on the higher temperatureside and HoCu₂ was used in the regenerator of the second stage, therefrigerating capacity at 4.2 K was 1.31 kW, and the lowest attainedtemperature with no application was 2.79 K.

50 wt % of the lower temperature side of HoCu₂ in the second stage wassubstituted with the regenerative materials such as Gd₂O₂S of example 8,the Zr-doped Gd₂O₂S to which ZrO₂ was added by 10 wt % (embodiment 24),the Al-doped Gd₂O₂S to which Al₂O₃ was added by 10 wt % (embodiment 25),or the Zr-doped Gd_(1.8)Tb_(0.2)O₂S to which ZrO₂ was added by 10 wt %(embodiment 28). Then, their refrigerating capacities were measured Theresults are shown in Table 21. The regenerative materials of theseembodiments exhibited refrigerating capacities and lowest achievedtemperatures which were substantially comparable with those of the rareearth metal oxysulfide regenerative materials to which no additive wasadded.

TABLE 21 Refrigerating Capacities Refrigerating capacity Lowest achievedSample Sample condition (4.2 K)/W temperature/K Prior art HoCu₂ 1.312.79 Example 8 Gd₂O₂S 1.69 2.62 Example 9 Gd_(1.8)Tb_(0.2)O₂S 1.73 2.61Embodiment 24 ZrO₂-10 wt % Gd₂O₂S 1.68 2.62 Embodiment 25 Al₂O₃-10 wt %Gd₂O₂S 1.68 2.63 Embodiment 28 ZrO₂-10 wt % 1.73 2.62Gd_(1.8)Tb_(0.2)O₂S

CONCLUSIONS CONCERNING ADDITIVES

As described above, what excel most in the durability under continuousoperation of refrigerators and the heat capacity are ceramics granulesto which any of Al₂O₃, Zro₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiCand Tic was added by 0.05–30 wt % in total, followed by ceramicsgranules to which any of alkaline earth metals (oxides of Mg, Ca, Sr andBa) and transition metal oxides (oxides of elements of which atomicnumbers are from 22 (Ti) through 31 (Ga) and 72 (Hf)) was added by0.05–30 wt %.

The description of the embodiments were centered mainly onGd_(x)Tb_(2−x)O₂S. Other rare earth metal oxysulfide regenerativematerials to which Al₂O₃, ZrO₂, mullite, Si₃N₄, Sialon, TiN, AlN, BN,SiC, or TiC was added by 0.05–0.3 wt % posed no problem even after10,000 hours of continuous operation of the GM refrigerator. When any ofalkaline earth metal oxides (oxides of Mg, Ca, Sr and Ba) and transitionmetal oxides (oxides of elements of which atomic numbers are 22 (Ti)through 31 (Ga) and 72 (Hf)) was added by 0.05–30 wt %, the embodimentsposed no problem after 2500 hours of continuous operation of the GMrefrigerator.

1. A rare earth metal oxysulfide regenerative material comprising a rareearth metal oxysulfide represented by a general formula R₂O₂S in which Rrepresents at least one member of rare earth elements selected from thegroup consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu and Y, characterized in that an additive comprising an oxide, acarbide or a nitride of at least one member selected from the groupconsisting of alkaline-earth metals, transition metals, and elements of3b and 4b groups of the periodic table excluding C is added to said rareearth metal oxysulfide by 0.05–30 wt %.
 2. A rare earth metal oxysulfideregenerative material of claim 1, characterized in that said rare earthmetal oxysulfide is Tb₂O₂S.
 3. A rare earth metal oxysulfideregenerative material of claim 1, characterized in that said additive isat least one compound selected from the group consisting of Al₂O₃, ZrO₂,mullite, Si₃N₄, Sialon, TiN, AlN, BN, SiC, and TiC.
 4. A rare earthmetal oxysulfide regenerative material of claim 3, characterized in thatsaid additive is at least one compound selected from the groupconsisting of ZrO₂, Si₃N₄, Sialon, TiN, AlN, BN, SiC, and TiC.
 5. A rareearth metal oxysulfide regenerative material of claim 1, characterizedin that said additive is an oxide of at least one alkaline earth metalelement selected from the group consisting of Mg, Ca, Sr, and Ba.
 6. Arare earth metal oxysulfide regenerative material of claim 1,characterized in that said additive is an oxide of at least onetransition metal element selected from the group consisting of elementsof which atomic numbers are 22 (Ti)–31 (Ga) and 72 (Hf).
 7. A rare earthmetal oxysulfide regenerative material of claim 1, characterized in thatthe rare earth metal oxysulfide regenerative material has a R₂O₂S phaseas a main phase and a second phase including said additive in a ceramicsmicrostructure, and the second phase is different from said main phase.8. A regenerator packed with a rare earth metal oxysulfide regenerativematerial comprising a rare earth metal oxysulfide represented by ageneral formula R₂O₂S (R denotes at least one member of rare earthelements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, DY, Ho, Er, Tm, Yb and Lu, as well as Y).
 9. A regenerator ofclaim 8, characterized in the HoCu₂ as a regenerative material workingat a relatively higher temperature and the rare earth metal oxysulfideregenerative material as a material working at a relatively lowertemperature are packed in layers.
 10. A regenerator of claim 8,characterized in that a Tb oxysulfide regenerative material is packed ina layer for working at relatively higher temperature and a Gd oxysulfideregenerative material is packed in another layer for working atrelatively lower temperature.
 11. A regenerator of claim 10,characterized in that a Ho or Dy oxysulfide regenerative material ispacked in a further layer on a lower temperature side of the Gdoxysulfide regenerative material layer.
 12. A regenerator of claim 8,characterized in that an additive comprising a compound of an oxide, acarbide or a nitride of alkaline-earth metals, transition metals, orelements of 3b and 4b groups of the periodic table excluding C is addedto said rare earth metal oxysulfide by 0.05–30 wt %.
 13. A regeneratorof claim 12, characterized in that said additive is at least onecompound selected from the group consisting of ZrO₂, Si₃N₄, Sialon, TiN,AlN, BN, SiC, and Tic.
 14. A regenerator of claim 8, characterized inthat said rare earth metal oxysulfide regenerative material isGd_(2−x)Tb_(x)O₂S (x=0–2).
 15. A regenerator of claim 14, characterizedin that the value of x in said rare earth metal oxysulfideGd_(2−x)Tb_(x)O₂S is 0.2–2.